First of all, where EXACTLY did you see this expression? Always give the context. I have found it listed in legal documents, written in French, however "le dictionnaire de l'Académie Française" said there was no such term. I even found the word "ouiement" in a legal document written in English! Small wonder that you could not find it.

Thinking of sirens, they were noted for their "singing" so could it be a synonym (from the middle ages?) for "le chant?"

It was not in the etymology dictionaries either, but I am still looking!